The production of organic chemicals by microorganisms is well known to those familiar with the fermentation art. Such fermentation reactions frequently produce a variety of products in dilute aqueous solutions. The expense of separating the chemicals from each other and from the large volume of water has been so great that production of chemicals by fermentation has not been able to compete with production of the same chemicals from fossil fuel sources. However, the gradual depletion of petroleum fossil fuel with the resultant increase in prices of petrochemical feedstocks has revived interest in such fermentation reactions which can convert carbohydrates that are renewable raw materials into simple organic chemicals.
Chemicals which can be produced by fermentation reactions include the aliphatic organic acids: acetic acid, propionic acid, and butyric acid. However, the microorganisms which produce these acids are most productive in growth media at or near a neutral pH. When attempts are made to run these fermentations at low pH where free acids are produced, the organisms do not grow well and only very low concentrations of the acids are obtained. As a result, the fermentations are run at a pH which gives mainly the salts of the acids rather than the free acids themselves.
It is very difficult to separate salts of the organic acids from the dilute aqueous fermentation media in which they are produced. Moreover, the free acids are the products of interest to the chemical industry. For this reason, mineral acids are generally added at the end of the fermentation in order to convert the acid salts to the free acids. This acidification produces quantities of inorganic salts as by-products which are of little value. Their disposal also increases the cost of the process.
A two-step fermentation has now been discovered wherein the second step is a biochemical acidification. The free acid is formed in the second step together with a lactate salt which can be used as the carbon source in the first step of the process. Thus, the present method produces the free acids in good yield and high concentration without giving the useless by-products formed in the prior processes.
Furthermore, the two fermentation steps of this new process are both anaerobic fermentations. Since no oxygen is used in these reactions, no substrate is lost due to oxidation. This provides efficient incorporation of the carbon of the starting material into the product acid. Thus, the process incorporates carbon of the starting material into the product and with the same efficiency as a single-step anaerobic fermenttion.